One way:

Select your cell(s)/column(s) - I'll assume column A, with A1 the
active cell. Choose Format/Conditional Formatting...

Set the dropdowns and input box to

Formula Is =A2<A1

click the Format button, and in the Borders pane, apply a bottom border.


In article ,
Mary wrote:

I am trying to create a template that has a line at the end of similar rows,
I said cells earlier, basically it is a list. So I might have 3 rows that are
like this:
Marker
Marker
Marker
Pen
Pen
I want a bold line between Marker and Pen, but I want the line to
automatically adjust for different data that is imported. Is this possible
and if so how?